Research On Deformation Theory And Characteristics Of Machined Surface For High-speed Milling Aviation Aluminum Alloy

High speed machining (HSM) is one of the advanced manufacturing technologies and has been become the important trend of modern manufacture technology and has wide application prospects because of higher production efficiency, higher machining accuracy, higher surface finish, and lower production cost. Since the industrialization and generalization of HSM in the end of 20 century, it has already been widely applied in aviation, automobile, dies and molds industries etc. and obtains enormous economic effects. Aviation manufacture industry is the earliest and comprehensive industry in applying high speed machining technology. Instead of conventional welding components, many main frame parts are the large-scale integrated parts to get excellent rigidity, which need high material removal rate, high surface finish and long machining period. So the application of HSM will perfect meet these requirements and improve the efficiency and machined quality in the machining of large-scale integrated parts with thin-walled, complicated structures and high machining accuracy in aircraft. However basic theories of machining, deformation theory and surface formation characteristics of high speed machining are different with those in conventional machining. In practical operations, there are numerous new problems arising for both technical and theoretical aspects which need to be urgently solved.The non-uniform thermo-mechanical coupling intense stress fields are occurred in workpieces adjacent to machining region due to the action of the deformation of high strain rate speed and teratology in high speed machining between tool and workpiece in HSM. The characteristics of flow stress, chip formation, cutting mechanics, cutting temperature and machined surface formation are different with conventional machining. The theoretical, experimental and application researches on deformation theories and surface characteristic of high speed machining aluminum alloy 7050-T7451 were carried out systematically in order to provide reliable and important theoretical and practical methods.Accurately mechanical characteristic model of workpiece in high speed machining is important foundation for machining simulation and analyzing machining formation process with analytical or FEM methods. However the mechanical characteristic model with high strain rate and large strain in high speed machining is very difficult to be obtained just by means of only conventional material method. The empirical Johnson-Cook constitutive model is proved to be much adaptable and compatible than other empirical models for aluminum alloy by the results of high temperature split Hopkinson pressure bar compression test. The orthogonal machining methods was proposed to build constitutive model and research the relationship of shear flow stress, shear strain and strain rate in high speed machining. The constitutive model for aluminum alloy 7050-T7451 was established and input to FEM simulation software as material model. The output values of cutting force between experiment and simulation are compared, the results prove that the proposed model with orthogonal cutting technique is valid and reliable.Investigations on the basic theory of deformation for high speed machining 7050-T7451 aluminum alloy are conducted, including chip formation, cutting mechanics, friction coefficient, etc. The contents mainly includes: 1) The microcosmic mechanics of chip formation has been studied, and the deformation zones of front edge and compression of dislocation are added after the deformation are re-divided into five zones with dislocation theory in order to describing deformation accurately in machining. 2) By theoretical analysis and experimental approaches, the milling force empirical models of three kinds of tools have been established. The milling parameters effect on milling force was studied, the results prove that the critical value of high speed for 7050-T7451 is 900m/min. 3) The average friction coefficient was obtained by the orthogonal cutting force tests, in high speed machining the various tendency of frication coefficient and frication angle with cutting speed have been studied. At last considering the effect of work piece and cutting speed the new shear angle model of aluminum alloy 7050-T7451 was established and proved to be more suitable for expressing the variety of shear angle in high speed cutting process.Based on three-dimensional cutting geometric models, and the key techniques of large deformation thermo-mechanical coupling theory, high speed machining aluminum alloy 7050-T7451 process is simulated with Deform-3D software. The simulations of chip formation, field of stress, strain, temperature are analyzed and researched, the stress field in high speed machining, which is formed non-uniform thermo-mechanical coupling intense stress fields is very different with that in conventional machining. In initial and stable stage of machining process, the cutting force of simulation output are compared with those obtained by cutting experiments, and the precision of the above simulation is also verified. These investigations provided a foundation for the subsequent works in analysis and research of high speed mechanisms.The characteristics of machined layer formation are researched systematically based on high speed machining experiments and theoretical analysis. The empirical model of surface roughness in high speed milling is established and the effects of cutting parameters on surface roughness are studied comprehensively. Based on micro hardness experiments and the thermo-mechanical coupling deformation theory, the work-hardening ability and depth of hardness of surface in high speed machining aluminum alloy 7050-T7451 are researched. In different cutting conditions the experimental results of frictional wear performance of surface show that the machined surface in high speed milling has higher surface quality and better performance of wear-resisting than that in conventional machining.The characteristic of machined surface of high speed machining aluminum alloy 7050-T7451 is researched by micrographic experiments (SEM and TEM) and analysis. The formation mechanics of machined metamorphism layer has been analyzed and studied. The dislocation-energy model in machined metamorphism layer was established and applied to explain the micrographic mechanics work-hardening using the thermo-mechanical coupling deformation theory and dislocation theory. The experimental results show that the interaction of high dislocation density effect on hardening is more remarkable than that of thermal stress in high speed machining.The project is supported by National Natural Science Foundation of China (Grant number 50435020)...